It sounds like it might be useful in a few instances. But, I hesitate to install add-ons unless it is something that I'm likely to use all the time. I'm afraid excess extensions will slow FF down.
I often see extensions that don't offer much extra functionality but still eat resources. Example: Flagfox puts a flag in the location bar to signify the location of the server. Neat, but I don't think it is worth the resources.
I could be all wrong here. Does anybody have more than about 9-10 extensions installed and does it slow FF down?